8/12/2009

Funky Science: Lava lamps

The Science Store at MOSI has a really huge lava lamp on display and lots of smaller lava lamps for sale right now. The large lava lamp is really awesome to watch so I thought perhaps we should talk about the science of lava lamps!

Lava Lamps: Lava lamps, also known as liquid motion lamps, have been around since the 60's. They are very cool to watch but there are some neat principles of science in action.

So how does a liquid motion lamp work? Liquid motion lamps require the use of two insoluble, near equal density liquids and a heat source used for adjusting the density of the liquids. Now lets break that down into bite sized pieces.

Liquids: One liquid forms the slow moving blobs, like in the picture, we will call this Red. The other liquid allows the blobs to float about, we'll call this liquid Purple.

These liquids must be immiscible, or mutually insoluble. This means that neither liquid will dissolve the other like oil and water and that Red and Purple will remain separate.

These liquids must be of a nearly equal density.

When turned off, liquid motion lamps appear to have two distinct layers of liquids . One is just slightly more dense than the other and lays on the bottom of the lamp. In our case the Red liquid is slightly more dense than the Purple liquid. Being more dense causes the Red liquid to sink to the bottom.

Heat: The heat source in a liquid motion lamp is generally a light bulb or lamp. As you likely know, light bulbs can get pretty hot when they have been turned on. When the lamp is turned on it heats up the slightly more dense Red liquid at the bottom of the lamp.

Changing the temperature of a compound is an easy way to change density. When a compound is heated the molocules in the compound spread apart making it less dense. As the density decreases, the compond become lighter and will rise above heavier, more dense compounds. For example, think of air: hot air rises and cold air sinks. Heated air becomes less dense and becomes "lighter" which causes it to rise above the more dense, "heavier" cool air.

Much like hot air, the Red liquid at the bottom of the lamp will become less dense as it is heated. This makes the Red liquid "lighter" and it begins to rise in blobs through the Purple liquid.

As they rise through the Purple liquid and away from the heat source, the Red blobs lose heat. This causes the molecules in the Red liquid to move closer together and this increases the density of the Red liquid. The Red blobs are become more dense than the Purple liquid and sink back to the bottom. At the bottom the Red liquid is warmed up again and the process repeats until you turn off the lamp and all the Red liquid cools down.

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